po: fix broken message extraction

Extracting separately the pot for the various languages and then
creating manually the global pot (by manually joining the above ones
after having stripped their headers) is wrong, since other than being
an hack it can create an invalid pot when the same message appears in
sources written in different languages.

Instead, a cleaner and safer solution is to first let ocaml-gettext
(if available) extract the messages for the ml files, and then use
xgettext to extract the messages for the other languages, joining the
new messages to the existing (or not) pot file.
fish: improve the command error messages

- when a command needs no parameters, tell that explicitly instead of
  "command should have 0 parameters"
- use gettext's plural form when printing the number of required
  arguments
- improve the error message for a variable number of parameters limited
  only in the maximum number of them, using also a plural form